The present invention relates generally to appliance devices, and more particular, to appliance devices having current sensor circuits.
Appliance devices such as dishwashers, clothing washing machines, dryers, ovens, refrigerators and the like often include electrical control circuits. Such control circuits receive input from the user and control the operation of the appliance device based on the received input. In many cases, the overall operation of the appliance is predefined as a general matter and the user input merely modifies the predefined operation in some way. For example, the operation of a dishwasher typically involves the processes of filling, washing, draining and rinsing. Such operations involve, among other things, the control of water valves, detergent valves and motor relays. The general sequence of such operations is generally predefined. However, user input may be used to alter the sequence, or to define certain parameters of the sequence. For example, the user input may define whether the wash cycle is normal, light, or heavy. Although the general sequence does not necessarily change dependent upon wash cycle selection, the length of certain processes within the sequence does change.
In addition, appliance control circuits often receive input from sensor devices in the appliance. The sensors provide feedback pertaining to appliance operation. For example, a dishwasher or clothes washer may include a temperature sensor, which allows the control circuit to regulate the temperature of the water. Appliances that have motors, for example, dishwashers, clothes washers and clothes dryers, will often include a current sense circuit that is used to sense the current level within one or more windings of the motor. The control circuit may then use the sensed current level for a variety of purposes.
One use of the current level pertains to the timing of energizing windings within the motor. In particular, many appliance motors include multiple windings that may be independently energized and de-energized. One winding may be a run winding that is energized during the steady state operation of the motor. Another winding may be a start winding that is energized during the start up of the motor. It is known that more energy is required to bring a motor up to steady state speed from a dead stop than that which is required to operate the motor at a steady state speed. Accordingly, it is also known to use start windings in addition to the run winding to provide extra driving energy to the motor during start-up. Once the motor reaches steady state, the start winding is de-energized.
One issue that arises from the use of start windings on an appliance motor is determining when to de-energize the start winding. If the start winding is de-energized too early, then the current in the main or run winding may increase dramatically, and may lead to inefficient operation or even winding damage. If the start winding is not de-energized, then the overall efficiency of the motor decreases because the losses increase as a function of the overall winding length.
One known method of controlling the de-energization of the start winding is to sense the current in the main winding of the motor. The current on the main winding of the motor varies inversely as a function of time from start-up. In other words, upon start up, the winding current is high and upon reaching steady state, the winding current is relatively low. Thus, the current sensor is used to determine when the winding current is low enough to correspond to steady state operation of the motor. When the control circuit, which receives a sensor signal from the current sense circuit, determines that the winding current is at a level consistent with steady state operation of the motor, the control circuit de-energizes the start winding.
One drawback associated with the use of current sense circuits is that they typically employ large circuit elements that add weight, cost, and manufacturing complexity. Because of the relatively high magnitude of motor current, low weight and low cost microelectronic devices are typically insufficient because they have limited current handling capacity. Accordingly, motor current sense circuits involve larger and more costly discrete components. In one example, prior art devices have employed large, coiled wire current sensing resistors as the main element of the current sense circuit. Such current sensing resistors were costly to both manufacture and assemble onto the circuit board. Because profit margins on appliances are relatively small, there is always a need to avoid costly components and manufacturing steps.
The present invention addresses the above needs, as well as others, by providing an appliance control apparatus that incorporates a current sense resistor as a trace on printed circuit board. The use of a circuit board trace as a current sense resistor reduces manufacturing complexity because the current sense resistor is formed at the same time as the other printed circuit board traces. Moreover, the cost associated with procuring a separate, coiled wire resistor is avoided.
An embodiment that incorporates this aspect of the invention is a motor control circuit that includes a first winding switch, a first switch driver circuit, and a current sense circuit. The first winding switch has a control input and is operable to activate a first motor winding. The first switch driver circuit is coupled to the control input. The current sense circuit is operably coupled to a second winding. The current sense circuit includes a sense resistor, the sense resistor comprising an etched trace in a printed circuit board. The etched trace has a geometry defining a resistance of the sense resistor.
Another embodiment that incorporates the current sense resistor of the present invention is an appliance control circuit arrangement that includes a current sense circuit and a controller. The current sense circuit is operably coupled to a winding of an appliance motor. The current sense circuit includes a sense resistor, the sense resistor comprising an etched trace in a printed circuit board. The etched trace has a geometry defining a resistance of the sense resistor. The controller is operable to obtain current sense signals from the current sense circuit, and generate a first signal responsive to the current sense signals obtained from the current sense circuit.
The above described embodiment not only may be used in situations in which a start winding is used to bring a motor up to speed, but also any situation in which a device is controlled in relation to the current flowing through the motor windings.
The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.